The present invention relates to a new process for curing aminoplastic resins; its subject matter is more particularly a new process for curing aminoplastic resins using a catalytic system making it possible to influence the reactivity of the aminoplastic resins.
Aminoplastic resins are known products which are widely employed in the woodworking industry, particularly for the manufacture of particle boards. The aminoplastic resins most commonly employed are urea-formaldehyde resins. They are manufactured in a known manner by condensation of urea and of formaldehyde at a pH of between 4 and 7 and at a temperature close to boiling point: this condensation reaction is preferably carried out in several stages.
The chief disadvantage of urea-formaldehyde resins is that they cause large quantities of free formaldehyde to be given off. Attempts have been made to reduce the free formaldehyde content by using various methods of manufacture; unfortunately, when the aim is to attain particularly low formaldehyde contents, experience shows that this objective is accompanied both by a decrease in the reactivity and the stability of the resins and by a deterioration in the mechanical properties of the finished boards. In order to get rid of free formaldehyde, it has also been proposed to employ resins devoid of formaldehyde, in particular resins based on isocyanate solutions. Unfortunately, the problem of formaldehyde is replaced by that of the isocyanates, whose action on man is more harmful and more durable than that of formaldehyde, since the presence of free isocyanate groups has been demonstrated in particle boards even several years after manufacture.
It has also been proposed to add melamine during the manufacture of the urea-formaldehyde resins. Unfortunately, the use of melamine with conventional urea-formaldehyde resins begins to be effective only starting at levels which may reach 20% and more, and this makes its use commercially prohibitive, given its cost. It has thus been proposed to lower the melamine content to values below 10%. A decrease in the reactivity of these resins is then observed, and this is particularly detrimental to maintaining commercially acceptable rates of manufacture.
There is therefore a need to have available urea-formaldehyde resins containing melamine, but which exhibit both good reactivity, good stability and reduced formaldehyde contents and whose use results in final particle boards exhibiting good mechanical characteristics.